Method of controlling interference from femtocells in consideration of macrocell

ABSTRACT

A method of controlling femtocell interference considering a macrocell is provided. The method of controlling the interference includes a method of setting transmission power of the femtocell and methods of avoiding the interference of the femtocell. The methods of avoiding the interference include a PRB rotation method, a PRB transmission power restriction method, a hybrid method and a UB rotation method. The method of setting the transmission power and the methods of avoiding the interference may be implemented independently or implemented in combination with each other. Hence, according to the present invention, the femtocell itself can control the interference without affecting an interface of an existing macrocell base station and without signaling through the interface with the macrocell.

CLAIM FOR PRIORITY

This application claims priority to Korean Patent Application No.10-2010-0133286 filed on Dec. 23, 2010 in the Korean IntellectualProperty Office (KIPO), the entire contents of which are herebyincorporated by reference.

BACKGROUND

1. Technical Field

Example embodiments of the present invention relate in general to amobile telecommunication technique, and more particularly, to a methodof controlling interference of a femtocell to have the femtocell itselfavoid the interference on a macrocell and also have the macrocell not beinterfered with by the femtocell at the time of initial installation andmanagement of the femtocell in an environment where the macrocell andthe femtocell overlap.

2. Related Art

Controlling inter-cell interference becomes an important issue in a caseof a 4^(th) mobile telecommunication system using an orthogonalfrequency division multiplexing (OFDM) technique, and in particular,solving interference between a macrocell and a femtocell becomes animportant issue in a heterogeneous network where the macrocell and thefemtocell overlap.

Frequency allocation schemes for the femtocell and the macrocell arebroadly classified into two types. One is a co-channel frequencyallocation scheme in which the macrocell and the femtocell share thesame frequency band, and the other is an orthogonal channel frequencyallocation scheme in which the macrocell and the femtocell use the fullfrequency band with no frequency overlapping between the macrocell andthe femtocell.

In the former case, although there is interference between the macrocelland the femtocell, a frequency use ratio is high. In the latter case,although there is no interference between the macrocell and thefemtocell in the full frequency band, the frequency use efficiency ispoor.

That is, in the case of the co-channel frequency allocation scheme,solving the interference between the macrocell and the femtocell becomesan important issue. In particular, there is a difference betweentransmission powers of respective base stations of the macrocell andfemtocell, and the interference from the macrocell to the femtocell isnegligible due to a relative distance, however, the interference fromthe femtocell to the macrocell may be problematic.

Techniques of controlling the interference between the macrocell and thefemtocell according to the related art may employ a coordination methodbetween the macrocell and the femtocell. Such a coordination methodallows finer tuning to be performed between the macrocell and thefemtocell. However, considering actual femtocell deployment situations(e.g., open/closed/hybrid), exchanging messages for coordination betweennumerous femtocells and the macrocell is not feasible in practice. Thecoordination method causes many signaling loads on the network, analgorithm for handling the signaling loads has a high level ofcomplexity, and femtocell costs increase significantly. Further, themacrocell must also be upgraded.

SUMMARY

Example embodiments of the present invention provide a method of settingtransmission power of a femtocell that allows the femtocell toautomatically set transmission power of the femtocell.

Example embodiments of the present invention also provide a method ofallowing a femtocell itself to control interference without affecting aninterface of an existing macrocell base station and without coordinationwith the macrocell through the interface with the macrocell.

In some example embodiments, a method of setting transmission power of afemtocell includes: (a) measuring or estimating received power (PM) froma macrocell base station; (b) multiplying the measured or estimatedreceived power from the macrocell base station by a linear coefficientα; (c) adding, to a value obtained in operation (b), a minimum value βof the transmission power of the femtocell when the measured orestimated received power from the macrocell base station is 0; and (d)selecting a minimum value between a value obtained in operation (c) anda maximum transmission power value of the femtocell, and setting, as thetransmission power of the femtocell, a maximum value between theselected minimum value and a minimum transmission power value of thefemtocell.

Here, in operation (a), the estimating of the received power from themacrocell base station may include receiving a position and atransmission power value of the macrocell base station, and estimatingthe received power from the macrocell based on the received value andthe position. In this case, the femtocell may receive the position andthe transmission power value of the macrocell base station from acomponent in a network performing automatic neighbor relation (ANR).

Here, in operation (a), the estimating of the received power from themacrocell base station may include allowing the femtocell to operate asa macrocell by setting a cell type of the femtocell to the macrocellduring a predetermined period, and allowing a nearby terminal to measurethe received power from the macrocell base station when the nearbyterminal is accessed to the femtocell and estimating the received powerfrom the macrocell base station based on the measured received power.

Here, the linear coefficient may be set by the scale of the macrocell.

In other example embodiments, a method of controlling interference of afemtocell includes: (a) dividing frequency resources allocated to thefemtocell into a plurality of physical resource block (PRB) groups eachhaving at least one PRB and then into n (n is a natural number not lessthan 2) rotation PRB groups each having at least one PRB group; (b)performing a service of the femtocell using a first rotation PRB groupamong the rotation PRB groups during a first period; and (c) performinga service of the femtocell using a second rotation PRB group during asecond period after the first period when a channel quality of the firstrotation PRB group during the first period is not greater than apredetermined threshold or when a quality of service (QoS) is notensured.

Here, in operation (c), determining whether the channel quality of thefirst rotation PRB group is not greater than the predetermined thresholdmay be based on channel quality information (CQI) notified from aterminal connected to the femtocell.

Here, the PRBs included in the first rotation PRB group and the PRBsincluded in the second rotation PRB group may not overlap each other.

Here, the PRBs included in the first rotation PRB group and the PRBsincluded in the second rotation PRB group may overlap each other atleast partially.

Here, the frequency resources allocated to the femtocell may include atleast one between a downlink frequency resource from the femtocell andan uplink frequency resource to the femtocell.

In still other example embodiments, a method of controlling interferenceof a femtocell includes: (a) dividing frequency resources allocated tothe femtocell into a plurality of PRB groups each having at least onePRB and then into n (n is a natural number not less than 2) powercontrol PRB groups each having at least one PRB group; (b) performing aservice of the femtocell by applying one of at least two transmissionpower stages to transmission power for each of the power control PRBgroups during a first period; and (c) performing a service of thefemtocell during a second period after the first period by decreasingthe transmission power stage (i.e., applying low transmission power) setin the first period for the power control PRB group with a degradedinterference level and maintaining or increasing the transmission powerstage (i.e., applying high transmission power) set in the first periodfor the power control PRB group with an improved interference levelafter measuring the interference for each of the power control PRBgroups during the first period.

Here, in operation (b), when the first period is one that performs theservice with initial transmission power of the femtocell, the highesttransmission power among the transmission power stages may be set as astage that can be used for all of the n power control PRB groups.

Here, in operation (c), when the power control PRB group to which thelowest transmission power stage is applied does not have an improvedinterference level even after the second period, the femtocell may notperform the service through the power control PRB group.

In yet other example embodiments, a method of controlling interferenceof a femtocell in which plural frequency bands capable of beingallocated by a service provider are present includes: (a) performing aservice of the femtocell using a first frequency band during a firstperiod; and (b) performing a service of the femtocell using a secondfrequency band during a second period after the first period.

Here, the method of controlling the interference of the femtocell may beapplied to a femtocell located in an inner cell region (ICR) of amacrocell using the same UB (Used Band; operation carrier).Alternatively, the method of controlling the interference of thefemtocell may be performed by determining that a location of a macrocellbase station is close to a location of the femtocell itself based oninformation about a macrocell included in ANR after the femtocellreceives a message from the ANR.

BRIEF DESCRIPTION OF DRAWINGS

Example embodiments of the present invention will become more apparentby describing in detail example embodiments of the present inventionwith reference to the accompanying drawings, in which:

FIG. 1 is a flow chart for explaining a method of setting transmissionpower of a femtocell in accordance with an example embodiment of thepresent invention;

FIG. 2 is a graph for explaining a method of setting transmission powerof a femtocell in accordance with an example embodiment of the presentinvention;

FIG. 3 is a flow chart for explaining a PRB rotation method amonginterference avoiding methods of a femtocell in accordance with anexample embodiment of the present invention;

FIG. 4 is a frequency resource management diagram over time forexplaining a PRB rotation method among interference avoiding methods ofa femtocell in accordance with an example embodiment of the presentinvention;

FIG. 5 is a flow chart for explaining a method of restricting PRBtransmission power among interference avoiding methods of a femtocell inaccordance with an example embodiment of the present invention;

FIG. 6 is a frequency resource and transmission power management diagramover time for explaining a method of restricting PRB transmission poweramong interference avoiding methods of a femtocell in accordance with anexample embodiment of the present invention; and

FIG. 7 is a flow chart for explaining an UB (used band) rotation methodamong interference avoiding methods of a femtocell in accordance with anexample embodiment of the present invention.

DESCRIPTION OF EXAMPLE EMBODIMENTS

Example embodiments of the present invention are disclosed herein.However, specific structural and functional details disclosed herein aremerely representative for purposes of describing example embodiments ofthe present invention, however, example embodiments of the presentinvention may be embodied in many alternate forms and should not beconstrued as limited to example embodiments of the present invention setforth herein.

Accordingly, while the invention is susceptible to various modificationsand alternative forms, specific embodiments thereof are shown by way ofexample in the drawings and will herein be described in detail. Itshould be understood, however, that there is no intent to limit theinvention to the particular forms disclosed, but on the contrary, theinvention is to cover all modifications, equivalents, and alternativesfalling within the spirit and scope of the invention. Like numbers referto like elements throughout the description of the figures.

It will be understood that, although the terms first, second, etc. maybe used herein to describe various elements, these elements should notbe limited by these terms. These terms are only used to distinguish oneelement from another. For example, a first element could be termed asecond element, and, similarly, a second element could be termed a firstelement, without departing from the scope of the present invention. Asused herein, the term “and/or” includes any and all combinations of oneor more of the associated listed items.

It will be understood that when an element is referred to as being“connected” or “coupled” to another element, it can be directlyconnected or coupled to the other element or intervening elements may bepresent. In contrast, when an element is referred to as being “directlyconnected” or “directly coupled” to another element, there are nointervening elements present. Other words used to describe therelationship between elements should be interpreted in a like fashion(i.e., “between” versus “directly between,” “adjacent” versus “directlyadjacent,” etc.).

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of the invention. Asused herein, the singular forms “a,” “an” and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprises,”“comprising,” “includes” and/or “including,” when used herein, specifythe presence of stated features, integers, steps, operations, elements,and/or components, but do not preclude the presence or addition of oneor more other features, integers, steps, operations, elements,components, and/or groups thereof.

Unless otherwise defined, all terms (including technical and scientificterms) used herein have the same meaning as commonly understood by oneof ordinary skill in the art to which this invention belongs. It will befurther understood that terms, such as those defined in commonly useddictionaries, should be interpreted as having a meaning that isconsistent with their meaning in the context of the relevant art andwill not be interpreted in an idealized or overly formal sense unlessexpressly so defined herein.

As used herein, a “terminal” may indicate a mobile station (MS), a userequipment (UE), a user terminal (UT), a wireless terminal, an accessterminal (AT), a terminal, a subscriber unit, a subscriber station (SS),a wireless device, a wireless communication device, a wirelesstransmit/receive unit (WTRU), a mobile node, mobile or other terms.Various example embodiments of the terminal may include, but not limitedto, a cellular phone, a smart phone having a wireless communicationfunction, a personal digital assistant (PDA) having a wirelesscommunication function, a wireless modem, a portable computer having awireless communication function, an imaging device such as a digitalcamera having a wireless communication function, a gaming device havinga wireless communication function, music storing and reproducing homeappliances having a wireless communication function, internet homeappliances capable of performing wireless Internet access and browsing,or portable units or terminals incorporating combinations thereof.

As used herein, a “base station” is generally a fixed or mobile point incommunication with the terminal, and may indicate a base station, a nodeB, an evolved node B (eNode-B), a base transceiver system (BTS), anaccess point, a relay, a femtocell, etc.

Hereinafter, preferred example embodiments of the present invention willbe described in detail with reference to accompanying drawings.

A method of controlling interference of a femtocell according to anexample embodiment of the present invention includes a method of settingtransmission power of the femtocell and methods of avoiding theinterference of the femtocell. The methods of avoiding the interferenceinclude 1) a method of rotating a physical resource block (PRB), 2) amethod of limiting transmission power of the PRB, 3) a hybrid method ofcombining methods 1) and 2), and 4) an UB rotation method. The method ofsetting the transmission power and the methods of avoiding theinterference may be implemented independently, or may be implemented incombination with each other.

Hereinafter, the methods of controlling the interference will bedescribed in order.

Method of Setting Transmission Power at Femtocell According to anExample Embodiment of the Present Invention

FIG. 1 is a flow chart for explaining the method of setting thetransmission power of the femtocell according to an example embodimentof the present invention.

Referring to FIG. 1, the method of setting the transmission power of thefemtocell according to an example embodiment of the present inventionmay include (a) measuring or estimating received power from a macrocellbase station (S110); (b) multiplying the measured or estimated receivedpower from the macrocell base station with a linear coefficient (S120);(c) adding, to a value obtained in the multiplying, the transmissionpower of the femtocell when the measured or estimated received powerfrom the macrocell base station is 0 (S130); and (d) selecting a minimumvalue between a value obtained in the adding and a maximum transmissionpower value of the femtocell, and setting, as the transmission power ofthe femtocell, a maximum value between the selected minimum value and aminimum transmission power value of the femtocell (S140).

Setting the transmission power of the femtocell performed according tooperations S110 to S140 can be represented by equation 1 below.Ptx(HeNB)=max(min(α·PM+β,Pmax),Pmin) [dBm]  [Equation 1]

where Ptx(HeNB) is the transmission power of the femtocell to beobtained according to operations S110 to S140 described above, and Pmaxis the maximum transmission power of the femtocell. Pmin is the minimumtransmission power of the femtocell and will typically be 0 dBm. PM isthe received power from the macrocell signal that is received from themacrocell to be measured or estimated. α is the linear coefficient thatcan be adjusted based on the size difference between macrocells orallows a slope of a power mapping curve to be changed. β is a parameterrepresented by dB and corresponds to a transmission power value of thefemtocell when the measured or estimated received power from themacrocell is 0 in the PM range covered by a dynamic range for powercontrol.

FIG. 2 is a graph for further explaining a method of setting thetransmission power of the femtocell in accordance with an exampleembodiment of the present invention, wherein the meanings of α and β canbe described with reference to FIG. 2. α indicates the slope, and βindicates the power value when the PM is 0. When β is 0, the value atPM=0 is equal to Pmin.

Ptx(HeNB) of the femtocell base station cannot exceed the maximum 20 dBm(because the femtocell coverage is limited), and is determined at 20 dBmor less using the PM that is estimated by the network layer or receivedfrom the macrocell base station to be measured.

Here, PM may indicate the received power from the macrocell measured bya terminal (e.g., a macro UE that will be described later) connected tothe macrocell, the measured received power from the macrocell when thefemtocell has a function of measuring the received power from themacrocell, or a value estimated by the network layer. In this case,there are three methods of obtaining the PM.

The first method is to have the femtocell receive, in aid of theautomatic neighbor relation (ANR) component, the location andtransmission power of the macrocell that may be under influence and toestimate the PM using a general equation or a given equation (e.g. pathloss calculation equation) through the location of the femtocell.

When the femtocell is installed or enters the initialization for otherreasons, the femtocell may receive information of the various macrocellbase stations from the ANR component several times, or may receive theinformation by grouping various macrocell base stations that may beunder influence at once. At this time, the femtocell may estimate the PMthrough the operations below.

Operation 1) The femtocell searches for the macrocell base station thathas the same UB as the femtocell among macrocell base stationinformation received from the ANR component. The femtocell calculatesthe location from its femtocell base station.

Operation 2) The femtocell estimates the PM of the macrocell basestation using the transmission power information of the macro station,the location information of the femtocell, and the location informationof the base station having the same UB in operation 1). For instance,the general equation or the given path loss equation may be used.

Operation 3) The transmission power of the femtocell can be determinedusing equation 1 based on the estimated PM.

The second method is to have the femtocell operate as the macrocell basestation during a predetermined period, have a nearby macro terminalaccess the femtocell, and have the corresponding macro terminal measurethe received power from the macrocell to estimate the PM. The secondmethod may be performed by the operations below.

Operation 1) The femtocell puts off setting the closed subscriber group(CSG) field of the master information block (MIB) at the time of initialinstallation and as necessary (i.e., the femtocell operates as themacrocell base station).

Operation 2) The macro UE outdoors near the femtocell may performregistration. In this case, the registration result may or may not besuccessful in accordance with the relation between the femtocell and themacro terminal. For example, the registration result may or may not besuccessful based on in which state the femtocell is (open/hybrid/closed)and whether the corresponding terminal is the macro terminal and CSGterminal. In the registration operation, the femtocell may collect PMfrom the macrocell base station. The femtocell is in an RRC_CONNECTEDstate even when the actual registration is not successful, so that thefemtocell may instruct the macrocell to measure the macrocell basestation corresponding to the UB of the measurement through a“measConfig” to ensure that the femtocell has the PM of the macrocellbase station, and may disconnect wireless connection of the macroterminal. The measurement may be controlled using the ANR information.

Operation 3) The transmission power of the femtocell of the equation 1is determined by the measurement obtained in operation 2) (i.e., PM ofthe equation). Operations 1 and 2 may be performed by one PM measurementfrom the macro terminal or may also be performed based on the PMmeasurements collected by the macro terminal during a predeterminedperiod. Various algorithms may be applied for the number of measurementsand related statistics. However, ICR, CERI or II, no region may bedetermined and a priority of the algorithm per region may be changedbased on such measurements.

Operation 4) When the transmission power to be applied is determined, aCSG field is set in the MIB and then the femtocell is made to operate asthe femtocell base station.

The operations described above may be performed not only at an initialinstallation of the femtocell but also on demand and decision of thefemtocell. Such operations may be referred to as training operations.Start and stop of the training operations, and the number and period ofmeasurements on the received power from the macrocell base station bythe M UE in the training operations may be tuned in accordance with thesituation.

The third method is to have the femtocell directly measure the receivedpower from the macrocell on the UB used by the femtocell.

The third method increases the unit price of the femtocell but caneliminate message exchange operations performed in the first method. Thethird method is to allow the femtocell base station to have somefunctions of the macro terminal (i.e., function of measuring themacrocell base station). Since the femtocell base station directlyperforms the measurement, the training period that is relatively long inthe second method is not required, and the macro terminal does notnecessarily approach the femtocell base station, unlike the firstmethod.

The three methods described above each have advantages anddisadvantages. Hence, combined methods may be utilized rather than onlyone method.

Hereinafter, an interference avoiding method among the methods ofcontrolling the interference according to example embodiments of thepresent invention will be described. In this case, there are threeassumptions on the interference avoiding method according to exampleembodiments of the present invention.

Assumption 1) When the macrocell does not use the full frequency band,preferred bands for the inner cell region (ICR) or the cell edge region(CER) may be present.

Assumption 2) When the femtocell does not need to use the full frequencyband or when the allocated QoS or the channel quality for the allocatedband is not ensured, a scheduler of the femtocell may apply, based onassumption 1, an interference avoiding method 1 to be described later.

Assumption 3) The downlink transmission power is determined by themethod of setting the initial power of the femtocell described above. Onthe other hand, determination of the uplink transmission power of theterminal is broadly performed through the closed-loop power control inproportion to the transmission power of the femtocell. The interferencemeasurement on the uplink may be performed per PRB by the base station.On the other hand, the downlink interference is measured by theterminal, and related information is delivered at an L2-L2 or L3-L3level to have the L3 of the femtocell collect such measurementinformation. The interference information on the uplink (G)PRB may beprovided back to the terminal. That is, all interference information tobe measured is collected at the radio resource management (RRM) of thefemtocell.

Interference Avoiding Method 1—PRB Rotation

FIG. 3 is a flow chart for explaining the PRB rotation method amonginterference avoiding methods of the femtocell in accordance with anexample embodiment of the present invention.

Referring to FIG. 3, the PRB rotation method among interference avoidingmethods of the femtocell in accordance with an example embodiment of thepresent invention includes (a) dividing frequency resources allocated tothe femtocell into a plurality of PRB groups each having at least onePRB and then into n (n is a natural number not less than 2) rotation PRBgroups each having at least one PRB group (S310); (b) performing aservice of the femtocell using a first rotation PRB group among therotation PRB groups during a first period (S320); and (c) performing aservice of the femtocell using a second rotation PRB group during asecond period after the first period when a channel quality of the firstrotation PRB group during the first period is not greater than apredetermined threshold or when the QoS is not ensured (S330).

In operation S310, the frequency resources allocated to the femtocellare divided into a plurality of PRB groups each having at least one PRBand then divided into n rotation PRB groups each having at least one PRBgroup.

FIG. 4 is a frequency resource management diagram over time forexplaining the PRB rotation method among interference avoiding methodsof the femtocell in accordance with an example embodiment of the presentinvention. The concept of the divided PRB groups and rotation PRB groupsin operation S310 will be described with reference to FIG. 4. there arePRB groups 0 to 11 each having one PRB, and the PRB groups 8, 9, 10 and11 belong to the first rotation PRB group 410, and the PRB groups 4, 5,6 and 7 belong to the second rotation PRB group 420.

Next, operation S420 will also be described with reference to FIG. 4.

Referring to FIG. 4, when four (G)PRBs are used while the femtocellperforms the service, the first PRB group 410 (having the PRBs 8, 9, 10and 11) is allocated during the first Time 1. However, when theallocated channel quality (e.g., CQI) is poor or the QoS is not ensured,the second rotation PRB group 420 (having the PRBs 4, 5, 6 and 7) isallocated during the second Time 2. Similarly, when the allocatedchannel quality is poor or the QoS is not ensured even during the secondTime 2, the third PRB group 430 (having the PRBs 0, 1, 2 and 3) isallocated during the third Time 3. That is, when the channel quality ofthe allocated PRB group (i.e., the first rotation PRB group) during theprevious period is poor or the QoS is not ensured, resources forperforming the service are changed to a different PRB group (i.e., thesecond rotation PRB group) during the next period.

The PRB rotation is employed for the following reasons. The preferredband of the macrocell is present as described in assumption 1), it israre to use the full frequency band (use of full capacity) in an actualenvironment, and in particular, the full frequency band will not be usedin the CER.

As such, the PRB rotation may be variously applied with the basic formas illustrated in FIG. 4. Since the PRB rotation method has an effect ofallowing the macrocell base station to avoid the allocated resourceregion, it is possible to prevent the femtocell base station from beingkept in a poor environment due to overlapping between specific resourceallocation and resource allocation of the macrocell base station. Suchmechanism may be applied not only to the downlink but also to theuplink. That is, the PRB rotation method may be applied to both thedownlink frequency resources and the uplink frequency resourcesallocated to the femtocell.

In addition, the PRBs included in the rotation PRB group applied duringthe first period and the PRBs included in the rotation PRB group appliedduring the second period may not overlap, or at least some thereof maybe configured to overlap. For example, referring to FIG. 4, the PRBsapplied during the first and second periods do not overlap, however,some PRBs may overlap in consideration of the frequency resourcemanagement (i.e., overlapping scheme). For example, when the PRBs 8, 9,10 and 11 are applied during the first Time 1, the PRBs 5, 6, 7 and 8may be applied during the second Time 2 to have the overlapped PRB 8.

Interference Avoiding Method 2—PRB Transmission Power Restriction

FIG. 5 is a flow chart for explaining a method of restricting the PRBtransmission power among interference avoiding methods of the femtocellin accordance with an example embodiment of the present invention.

Referring to FIG. 5, the method of restricting the PRB transmissionpower among interference avoiding methods of the femtocell in accordancewith an example embodiment of the present invention includes (a)dividing frequency resources allocated to the femtocell into a pluralityof PRB groups each having at least one PRB and then into n (n is anatural number not less than 2) power control PRB groups each having atleast one PRB group (S510); (b) performing a service of the femtocell byapplying one of at least two transmission power stages to transmissionpower for each of the power control PRB groups during a first period(S520); and (c) performing a service of the femtocell during a secondperiod after the first period by decreasing the transmission power(i.e., applying low transmission power) set in the first period for thepower control PRB group with a degraded interference level andmaintaining or increasing the transmission power (i.e., applying hightransmission power) set in the first period for the power control PRBgroup with an improved interference level after measuring theinterference for each of the power control PRB groups during the firstperiod (S530).

First, in operation (a) S510, in a similar way to operation (a) S310 ofthe interference avoiding method 1 described above, frequency resourcesallocated to the femtocell are divided into a plurality of PRB groupseach having at least one PRB and then into n (n is a natural number notless than 2) power control PRB groups each having at least one PRBgroup.

In operation S520, one of at least two transmission power stages isapplied to the transmission power for each of the power control PRBgroups during the first period (Time 1) to perform the service. At thistime, the transmission power stages include at least two stages,preferably, three levels of high, medium and low.

In this case, levels of the transmission power stage may be determinedbased on the transmission power (Ptx(HeNB)) obtained by the transmissionpower setting method of the femtocell according to an example embodimentof the present invention. For example, the levels may be determinedusing fixed power values (for example, when the fixed power value is 5and the Ptx(HeNB) is 19, Ptx(high) is 19, Ptx(medium) is 14, andPtx(low) is 9), and a difference between the determined Ptx(HeNB) andthe Pmin is divided by the value (level(3)+1) to be used as the levelchange value. For example, in the case of three levels of high, mediumand low, (3)+1=4, and when the determined Ptx is 20, the level changevalue may be determined as ((Ptx(20)−Pmin(0))/4, that is, 5). In thiscase, Ptx(high) is 20, Ptx(medium) is 15, and Ptx(low) is 10.Alternatively, when the determined Ptx is very small to be close to thevalue (i.e., Pmin), Ptx(high), Ptx(medium) and Ptx(low) may not bediscriminated. In this case, there may be two transmission power stages,or the interference avoiding method 1 may be more preferred to theinterference avoiding method 2.

Next, in operation S530, the transmission power stage during the nextperiod is changed and applied for each of the power control PRB groupsafter the interference for each of the n power control PRB groups ismeasured during the first period.

For example, the transmission power set in the first period for thepower control PRB group with a degraded interference level is decreased(i.e., low transmission power is applied), and the transmission powerset in the first period for the power control PRB group with an improvedinterference level is kept or increased (i.e., high transmission poweris applied), thereby performing a service of the femtocell during asecond period after the first period.

Next, operation (b) S520 and operation (c) S530 will be described withreference to FIG. 6. Referring to FIG. 6, the transmission power stagesinclude three stage levels of high, medium and low.

FIG. 6 is a frequency resource and transmission power management diagramover time for explaining a method of restricting the PRB transmissionpower among interference avoiding methods of the femtocell in accordancewith an example embodiment of the present invention.

Referring to FIG. 6, it is assumed that one PRB forms one PRB group andone PRB group is allocated to one power control PRB group, and the PRB,the PRB group and the power control PRB are in one-to-one correspondencewith each other.

Referring to FIG. 6, to perform the service of the femtocell, PRBs 2, 3,4, 5, 6, 7, 8, 9, 10 and 11 are allocated for the Time 0 to use 10(G)PRBs, and Ptx becomes Ptx(high) as an initial power in equation 1.

At this time, it is assumed that the PRBs 2, 3 and 4 are measured withhigh overloads and PRBs 5, 6, 7, 8, 9, 10 and 11 are measured with lowoverloads when the interference (overload) is measured for the (G)PRB atTime 1. As a result, the transmission power is changed to thePtx(medium)(Ptx(high)—specific value (1)) for the former PRBs 2, 3 and4.

It is assumed that the PRBs 2 and 4 are measured with medium overloads,the PRB 3 is measured with a high overload and the PRB 8 is newlymeasured with a high overload while the powers for the PRBs 2, 3 and 4are set as Ptx(medium) at Time 2. As a result, the transmission power ofthe PRB 3 may be changed to Ptx(low)(Ptx(high)—specific value (2)) andthe transmission power of the PRBs 2 and 4 may be selectively changed orkept (referring to FIG. 4, the previous power value was kept asPtx(medium) for the PRB 2 and changed to Ptx(high) for the PRB 4). ThePRB 8 is newly measured with a high overload, so that the transmissionpower is changed to Ptx(medium).

At Time 3, the PRB 3 is measured with a high overload even when thetransmission power has been adjusted to Ptx(low). In this case, other(G)PRBs such as PRB0 or PRB1 may be allocated to have Ptx(high) asdescribed in 2.2.1.

Such a mechanism can finely adjust the determined Ptx for each of the(G)PRBs when the femtocell base station uses the high capacity ofresources, so that the interference between macrocell base stations canbe finely tuned, and the macro terminal located in the CER of themacrocell can be prevented from being located in the femtocell coveragehole due to a relatively strong power of the femtocell.

The problem that the macro terminal located in the CER suffers from thecoverage hole due to the strong power of the femtocell located in theCER of the macrocell base station described above can be solved by asolution that allows the femtocell to more finely classify thetransmission power stages to decrease the (G)PRB Ptx in which the QoScan be implemented. The solution can solve the coverage hole problem ofthe macrocell due to the femtocell located in the CER.

Interference Avoiding Method 3—Hybrid Method

The femtocell may perform the interference avoiding method by combininginterference avoiding method 1 of changing the resource in terms of thescheduler with interference avoiding method 2 of adjusting the powerlevel in terms of the overload per (G)PRB.

For example, interference avoiding method 1, that is, the PRB rotationmethod, is performed referring to the QoS or CQI report from theterminal during a predetermined period. At this time, the CQI may be setfor the wideband (full band) or a specific subband (specific (G)PRB) sothat the CQI may be utilized for the (G)PRB rotation. However, when theoverload per (G)PRB can be measured, the (G)PRB rotation may beperformed only on the PRBs with the overload as described ininterference avoiding method 2. When the QoS is not satisfied in the PRBrotation method during the predetermined period, interference avoidingmethod 2, that is, the PRB transmission power restriction method, may beused.

On the other hand, interference avoiding method 2 may be used first, andthen interference avoiding method 1 may be used when the QoS is notsatisfied by interference avoiding method 2.

Such a combined method may be implemented using a specific rule ratherthan in a sequential manner. For example, the UB may be broadly dividedinto three parts, wherein interference avoiding method 1 andinterference avoiding method 2 (or interference avoiding method 2 andinterference avoiding method 1) may be sequentially performed for thefirst part, interference avoiding method 1 and interference avoidingmethod 2 (or interference avoiding method 2 and interference avoidingmethod 1) may be sequentially performed for the second part when aproblem occurs in the first part, and interference avoiding method 1 andinterference avoiding method 2 (or interference avoiding method 2 andinterference avoiding method 1) may be sequentially performed for thethird part when another problem occurs. Such methods may be performed inan overlapping manner. This is because the frequency band to be usedmight have been changed by the fractional frequency reuse (FFR)mechanism of the macrocell base station.

However, if the terminal connected to the femtocell is not provided withthe satisfactory service even by all such methods described above, thecorresponding terminal may be outbound-handed over to another basestation. On the other hand, when it is determined that the QoS is commonto most terminals connected to the femtocell, the correspondingfemtocell may perform the treatment on all connected terminals (e.g.,disconnection or outbound handover) and enter an initializationinstallation operation.

Interference Avoiding Method 4—UB Rotation Method

FIG. 7 is a flow chart for explaining an UB (Used Band; operationcarrier) rotation method among interference avoiding methods of thefemtocell in accordance with an example embodiment of the presentinvention.

Referring to FIG. 7, the UB rotation method among interference avoidingmethods of the femtocell in accordance with an example embodiment of thepresent invention is a method of controlling the interference of thefemtocell in which plural frequency bands capable of being allocated bya service provider are present, and the method includes (a) performing aservice of the femtocell using a first frequency band during a firstperiod (S710); and (b) performing a service of the femtocell using asecond frequency band during a second period after the first period(S720).

That is, as long as the femtocell is capable of changing the UB, in asimilar way to the interference avoiding method 1 (PRB rotation method),the UB itself can be changed by the interference avoiding methods 1, 2,and 3 described above even when the strong interference occurs.

For example, when a service provider performs frequency allocation (FA)such that the allocated frequency bands 1-30 are divided into thefrequency bands 1-10 (A), 11-20 (B) and 21-30 (C) so as to use thecells, automatically changing the UB itself by changing the UB in ansequential order of A, B and C at the femtocell base station may be themost preferred method for the femtocell base station located in the ICRof the macrocell base station using the same UB.

In addition, when the location of the macrocell base station is veryclose to the location of the femtocell itself based on the informationabout the macrocell included in message information after the femtocellreceives the message information from the ANR component (i.e., when themacrocell base station is located in the ICR of the base station 1), itmay be a very fast interference control method to immediately change theUB as long as the femtocell has the function of changing the UB.

In this case, when the macrocell base station is located in the ICR, itis possible to use the Pmax as the transmission power of the femtocell.In other words, even when the femtocell transmits the power of Pmax, theICR macrocell station and other macrocell stations are not affected.

According to the methods of controlling the interference of thefemtocell described above, the femtocell itself can avoid theinterference on a macrocell, and the macrocell cannot be interfered withby the femtocell at the time of initial installation and management ofthe femtocell in an environment where the macrocell and the femtocelloverlap. In particular, the femtocell itself can control theinterference without affecting an interface of an existing macrocellbase station and without coordination with the macrocell through theinterface with the macrocell.

While the example embodiments of the present invention and theiradvantages have been described in detail, it should be understood thatvarious changes, substitutions and alterations may be made hereinwithout departing from the scope of the invention.

What is claimed is:
 1. A method of controlling interference of afemtocell at a femtocell base station, comprising: (a) dividingfrequency resources allocated to the femto cell into a plurality ofphysical resource block (PRB) groups each having at least one PRB anddividing the plurality of PRB groups into N power control PRB groupseach having at least one PRB group, wherein N is a natural number notless than 2; (b) performing a service of the femtocell by applying oneof at least two transmission power stages to transmission power for eachof the power control PRB groups during a first period; and (c)performing the service of the femtocell during a second period after thefirst period by decreasing the transmission power stage set in the firstperiod for the power control PRB group with a degraded interferencelevel and maintaining or increasing the transmission power stage set inthe first period for the power control PRB group with an improvedinterference level after measuring the interference for each of thepower control PRB groups during the first period.
 2. The method of claim1, wherein, in operation (b), when the first period is one that performsthe service with initial transmission power of the femtocell, a highesttransmission power among the transmission power stages is set as a stagethat can be used for all of the n power control PRB groups.
 3. Themethod of claim 1, wherein the at least two transmission power stagescomprise at least three stages of high, medium and low.
 4. The method ofclaim 1, wherein, in operation (c), when the power control PRB group towhich the lowest transmission power stage is applied does not have animproved interference level even after the second period, the femtocelldoes not perform the service through the power control PRB group.
 5. Themethod of claim 1, wherein the method of controlling the interference ofthe femtocell is performed at the femtocell base station, withoutcoordination with a macrocell base station.